Printing office machines, for example typewriters, bookkeeping machines and the like, are known in which paper is supplied from a stack held in a cassette. The cassette normally contains a pressure plate which is spring-loaded by compression springs situated at its rear side and urges the ream of paper against a driven separating transport roller. The cassette also includes frontal paper handling tabs which are spring-loaded and touch the top of the stack of paper. When the separating roller grasps the top sheet of the stack, the sheet is forced against the corner tabs from which it is forced out by a snap action and thus separated from the rest of the stack for transport to the platen of the office machine.
It is common practice to place the cassette in the apparatus in the horizontal position. In that case, the corner separating mechanisms can be permitted to lie on the top of the paper stack due to their own weight or perhaps aided by a relatively weak spring. The stack of paper is urged upwardly against the separating roller by the spring-loaded pressure plate. The compression spring which provides the upward force is highly compressed at the outset when the cassette contains a full stack of paper and thus its pressure is relatively high. However the pressure exerted by the spring is partially compensated by the weight of the stack of paper which is initially high and decreases as the cassette is being emptied. At the same time, the force of the compression spring decreases so that the paper is pressed against the separating roller with a nearly constant force during the operation of the machine for any degree of filling of the cassette and this kind of constancy is desirable from the point of view of paper transport.
It is a disadvantage of any horizontal cassette holder that the space required for the insertion of the cassette is relatively large.
This disadvantage can be overcome by a vertical cassette position which however entails other problems, for example, the fact that the stack of paper stands substantially vertically on its small edge and rests in the apparatus of the prior art on the lower section of the corner separating mechanisms. The mobility of these corner separators is thus impeded by the relatively high initial weight of the paper stack. In order to insure that the corner separating mechanisms are always reliably in contact with the paper stack in spite of the weight which they have to support, it is generally necessary to provide relatively high spring loading of these mechanisms. However it is a consequence of the relatively high spring forces that when the paper stack decreases in size and thus becomes lighter, the corner separating mechanisms are pressed against the paper stack with great force and thus make the snap-out of the top sheet more difficult. Similarly, the compression spring which urges the pressure plate against the separating roller must be relatively strong so as to insure that the heavy paper stack which rests on the corner mechanisms is pressed sufficiently well against the separating roller. As a consequence, when the cassette is being emptied and the weight of the stack is reduced the entire stack is pushed with great force against the separating roller. This high contact presure makes it difficult to transport the top sheet around the writing platen of the machine and this difficulty tends to cause problems in the line feed mechanism of the typewriter or the like.
The latter difficulty is treated by an improvement disclosed in the German Offenlegungsschrift No. 27 15 649 by an additional mechanism which insures that the pressure plate bears against the paper stack only during the initial pickoff of the top sheet but pivots out of the way when the top sheet of paper is pulled through the machine by the printing platen. Accordingly, the relatively high pressure bearing against the separating roller does not impede the transport of the top sheet of paper by the printing platen. However, the additional mechanism required for the disabling of the pressure plate is relatively complicated both in construction and operation and is subject to malfunctions.